A satellite signal is usually intended to be received and processed by a plurality of receiver/processors, and the electronic components and/or processing paths of these receiver/processors may differ substantially from each other, even where the receiver/processors are produced by a single manufacturer. Each receiver/processor will filter an incoming signal as part of the signal processing, and the electrical parameters (impedances, capacitances, inductances, active component values) of the circuit that produces the filtered signal in one receiver/processor will differ from the corresponding electrical parameters in another receiver/processor. If the incoming signal is characterized with a known and fixed frequency spectrum, the resulting filtered signal for any two receiver/processors will differ from each other. Further, the output signal spectra produced by transmitters in two satellites may also differ from each other, because the two satellites are manufactured differently or because the two transmitters' component values differ.
An important example of these differences occurs in processing of Global Positioning System (GPS) signals received at one or more GPS signal antennas from two or more GPS satellites. Each GPS antenna is connected to one or more GPS signal receiver/processors that receive the satellite signals and determine the pseudorange, or length of time required for a satellite signal to propagate from the satellite to antenna. A GPS signal-transmitted by a single GPS satellite and received and processed at two equivalent GPS receiver/processors will produce two slightly different receiver/processor output signals. The same GPS signal, transmitted from each of two GPS satellites and received and processed at a single GPS receiver/processor, will produce two slightly different receiver/processor output signals.
Further, input signals received from two GPS satellites may differ qualitatively from each other because one satellite signal contains P-code and the other satellite signal is either a C/A code signal or is processed using cross-correlation analysis or another suitable processing technique. In such an instance, the two input signals are processed differently by a given receiver/processor and produce different output signals.
Two consequences of these differences are that: (1) the output signals produced by two receiver/processors, located at the same site, from an input signal received from a single satellite, will differ from each other; and (2) the output signals produced by a single receiver/processor from input signals received from two satellites may differ from each other. Ideally, two receiver/processors, located at the same site and receiving identical input signals from any satellite, should produce identical corresponding filtered output signals no matter which satellite transmitted the signals.
Workers in other fields have occasionally confronted persistent, time-dependent signal errors and have devised various approaches for reducing the effects of these errors on the system of interest. Hawkins, in U.S. Pat. No. 4,302,666, discloses determination of the effect of aiming errors in a rocket launcher, which includes coarse aiming and fine tune aiming, by computing a time average of the magnitude and the direction of the aiming errors. The rocket launcher is re-aimed after each launch, based on the perceived errors.
In U.S. Pat. No. 4,345,206, Skalka discloses formation of a difference between a machine-counted value, such as number of oscillator pulses, and the "true" value over a selected time interval to determine an average count error. The time interval length used for comparison is varied according to the count difference.
A time averaging, signal detection circuit that increases the signal-to-noise ratio (SNR) of a received signal is disclosed by Fothergill in U.S. Pat. No. 4,498,052. A received signal is fed to a first operational amplifier, then through a transformer, and the transformer output signal is partly subtracted from the op amp input signal. The op amp output signal has an enhanced d.c. and improved SNR.
Gigandet et al disclose a cooking oven system that computes and displays the average baking time of some food article only if the magnitude of the difference between the average baking time and a reference baking time is at least equal to a selected threshold, in U.S. Pat. No. 4,615,014. This difference is used to control speed of a conveyor belt that carries food articles through the oven.
A demodulator for PSK-modulated signals with a possibly time-varying carrier frequency is disclosed in U.S. Pat. No. 4,827,488, issued to Shibano. The time average of the frequency of an arriving signal is computed and taken as the present value f.sub.c of the carrier frequency. A mixer then mixes the arriving signal with the output signal from a variable frequency local oscillator with frequency f.sub.c to produce a square wave output signal that is used to reconstruct the data modulated onto the carrier wave. A pulse counter counts pulses in the square wave over a selected time interval and subtracts this number from a reference count number. The subtractor output signal is converted to an analog signal that controls the local oscillator frequency f.sub.c.
U.S. Pat. No. 5,206,500, issued to Decker et al, discloses a pulsed laser detection system with noise averaging. A photodetector (.phi.d) output signal is converted into (1) a first electrical signal with increased pulse width relative to the .phi.d output signal and into (2) a second signal representing the time average of noise in the .phi.d output signal. A third signal is formed that varies linearly with the difference of the first and second signals. A first comparator circuit compares the difference between the magnitude of the first signal and the third signal and generates a first comparator output signal. A second comparator circuit compares the difference between the magnitude of the .phi.d output signal and the magnitude of the third signal and generates a second comparator output signal. If the first comparator output signal exceeds a first selected threshold and the second comparator output signal does not exceed a second selected threshold, the system determines that a (laser) pulse is present.
Lippel, in U.S. Pat. No. 5,253,045, discloses a television imaging system that interpolates finer intensity levels in critical areas of a dither-quantized television picture that is transmitted or received with relatively few intensity levels. In one embodiment, time averages over several consecutive frames of pixel regions that are not changing much with time are substituted for the present images on these pixel regions. Switching between these time-averaged image regions and the present images in these image regions is available.
An analyzer for a multi-cylinder engine that forms time averages of time intervals between firings of each cylinder and firings of the next consecutive cylinder is disclosed in U.S. Pat. No. 5,258753, issued to Jonker et al. The fractional differences of these cylinder-to-cylinder intervals are computed, displayed and used to analyze engine performance.
In U.S. Pat. No. 5,291,284, issued to Carr et al, a predictive coding/decoding scheme with error drift reduction is disclosed. The tendency of long-term or time-averaged bias error to accumulate is reduced by periodically reversing the polarity of the estimated bias error relative to the estimate of the desired signal.
Signal differences that arise in a Global Positioning System (GPS) have been analyzed and used by a few workers. In U.S. Pat. No. 5,220,509, Takemura et al disclose a vehicle navigation system that uses GPS signals and signals from another self-contained navigation system (SCNS), such as a geomagnetic sensor. If the magnitude of the difference between a GPS-determined variable, such as bearing, and the corresponding SCNS-determined variable is greater than a selected threshold value, the SCNS value is corrected, using the GPS value. SCNS-determined navigation information is used exclusively when vehicle velocity is below a selected threshold (about 10 km/hour), where GPS navigation information is likely to be less accurate for a moving vehicle.
The signal differences formed and analyzed in the patents discussed above do not focus on the effects of differences in signals formed by two ostensibly identical signal receivers or on the effects of differences in processing of signals containing the same information. What is needed is a procedure that can be used to compensate for the differences in the corresponding filtered signals produced by two receiver/processors that perform the same functions. Preferably, the procedure should not depend strongly upon whether the receiver/processors are the same model or different models or upon whether the receiver/processors are manufactured by the same or different manufacturers. Preferably, the procedure should be flexible enough to take account of subsequent changes in the character of the incoming signals. Preferably, the procedure should take account of the possibility that two signals with similar information content can be processed differently by two receiver/processors to produce quantitative values for the same variables of interest.